Galindo, V. et al., "A Near-Isotropic Circulary (sic) Polarized Antenna for Space Vehicles", IEEE Transactions on Antennas and Propagation, Vol. AP-13, No. 6, November 1965, pp. 872-877, discloses a near isotropic circular polarization (CP) antenna for spacecraft and the like featuring crossed slot radiators in a cylindrical waveguide, one end of which is sealed by a short circuit. The present invention offers the following advantages over the Galindo device:
1. The radiating means of the present invention (comprising slots, a dielectric, and conductive patches) is mechanically stronger than the Galindo radiating means, which comprises cross slots.
2. The radiating means of the present invention offers more design flexibility than Galindo's cross slots, because the ratio of the dimensions of the patches can be varied; the thickness and dielectric constant of the dielectric can be varied; and the shape and orientation of the slots can be varied. In Galindo, just the shape of the cross slots can be varied.
3. In the present invention, the distance between the short circuit and the radiating means can be varied to improve the impedance match Galindo's device cannot accommodate this technique, because of the requirement for certain currents at the slots.
4. The Galindo device does not show an inner waveguide (e.g., for receive) within an outer waveguide (e.g., for transmit) as in the present invention.
5. The Galindo device does not disclose a reflecting frusto-cone as in the present invention.
U.S. Pat. No. 4,197,549 discloses a slot antenna for UHF broadcasting having a coaxially fed waveguide that generates either linearly or circularly polarized radiation. The reference antenna is suitable for radiating broadside beams but not for endfire beams as required for the instant spacecraft application. The reference uses slot radiators rather than the slot-fed patch radiators of the present invention.
U.S. Pat. No. 4,297,706 also discloses an antenna that is suitable for broadside radiation but not for endfire radiation. In fact, one of the goals of the reference device is to eliminate endfire radiation. The present invention uses patch radiators as opposed to the slot radiators of the reference.
U.S. Pat. No. 4,527,163 discloses yet another antenna which is not designed to radiate endfire (referred to as zero degrees elevation angle in FIGS. 6, 9 and 12). The word "omnidirectional" as used in the reference refers to the azimuthal plane only.
In the reference, CP is generated in the far field by means of a horizontal radiator and a vertical radiator that are fed 90 degrees out of phase. The present invention, on the other hand, generates CP in the far field broadside, via individual patch radiators, and axially, via a pattern of constructive interference.
In the reference, the radiators are fed with linear polarization. In the present invention, the transmit waveguide is fed with CP.
In the reference, the diameter of the cylindrical waveguide is restricted to be about 1/12 of a wavelength. In the present invention, this diameter is about 12 times larger.
A near-isotropic antenna was built by the assignee of the instant patent application and incorporated into the INTELSAT V series of satellites more than one year before the filing date of the instant application. In this antenna, the radiating means comprised slots, not patches as in the present invention. The INTELSAT V antenna produced circular polarization endfire (axially along the cylinder), but linear polarization broadside. The present invention, on the other hand, gets circular polarization both endfire and broadside, because it is a near-isotropic radiator By this is meant that electromagnetic energy surrounding the antenna is circularly polarized over a substantial portion of any sphere having a center positioned at the center of the cylindrical end of the antenna.
A similar slotted-cylinder reflector antenna using a reflecting cone is described in Y.T. Lo and S.W. Lee, Antenna Handbook, Chapter 21 "Satellite Antennas," by C.C. Han and Y. Hwang (published in 1988 by Van Nostrand Reinhold). Like the Intelsat V antenna, this antenna does not have radiating patches; and linear polarization is generated broadside, CP axially.